The present invention relates to pulverized fuel-fired steam generators and, more particularly, to an improved apparatus for disposing of pyrites and other pulverizer rejects.
Coals found in the United States and presently being fired in pulverized fuel-fired steam generators may contain up to 50 percent non-combustible material termed ash. A portion of this non-combustible material is extremely hard and resists pulverization as the coal is ground to a powder in the pulverizer and swept by an air stream into the furnace to be combusted therein. This portion of non-combustible material which resists pulverization, primarily iron pyrites and tramp iron, is separated from the pulverized coal by screening and rejected from the pulverizer. The remainder of the non-combustible material contained in the raw coal is effectively pulverized and passes to the furnace with the pulverized fuel and typically collects either on the walls of the furnace or on the downstream convection surface as ash deposits.
In a typical dry-bottom pulverized coal-fired steam generator, a water-filled bottom ash hopper is disposed directly below the furnace to collect and hold for subsequent removal ash deposits which break away from the furnace walls during the wall cleaning cycle. Since the bottom ash hopper is a readily accessible storage receptacle, it is considered most economical to the pulverizer rejects to the bottom ash hopper for storage instead of providing an additional separate system for disposing of the pulverizer rejects.
In the typical prior art pulverizer rejects disposal system, rejects from the pulverizer are collected in a small hopper next to the pulverizer. The rejects flowing from the collection hopper are mixed with water and conveyed as a slurry through a sluice pipe by means of a high pressure jet pump and injected into the water-filled bottom ash hopper. Attempts to utilize the bottom ash hopper as a concomitant storage as just described has encountered a major problem which is generally lead to the abandonment of this approach in favor of a separate storage receptacle for the pulverizer rejects themselves. As the water-filled bottom ash hopper is located immediately below the furnace hopper, the waterwall tubes formed in the furnace hopper slope pass over a portion of the bottom ash hopper on their way to lower water-wall inlet headers. Consequently, these tubes, which are near saturation temperature during furnace operation, are exposed to the cool water filling the bottom ash hopper which is typically at a temperature of 140.degree. F. to 160.degree. F.
In order to eliminate the possibility of cool water splashing against the hot waterwall tubes disposed directly above the bottom ash hopper, it was common to inject the pulverizer rejects/water slurry into the bottom ash hopper at a point below the water level therein. However, a major problem arose as a result of air entrained in the slurry when the rejects from the air-swept pulverizer were mixed with the water to form the slurry. The entrained air would bubble violently upward out of the bottom ash hopper thereby carrying cool water onto the hot tubes exposed above the bottom ash hopper. The repeated thermal shock resulting therefrom causes an unacceptable frequency of tube failure in the furnace hopper tubes and leads to the abandonment of the use of the bottom ash hopper as a storage receptacle for pulverizer rejects.